Capacitive intrusion detection system with balanced resonant circuits

ABSTRACT

Each of a pair of detector wires is strung along a portion of a boundary to be protected so that both wires are exposed to similar general environmental conditions. The wires are interconnected in respective resonant circuits which are similarly driven at a common frequency so that each resonant circuit provides an a.c. signal having a phase and amplitude which are variable as a function of the capacitance of the respective detector wire. By synchronously demodulating the difference between the two signals, a difference amplitude signal is obtained which is relatively sensitive to differential changes in the capacitances. By employing the rate of change of this demodulating difference signal as an indication of intrusion, a system is obtained which is relatively insensitive to gradual and environmental changes.

United States Patent Ralston 1 Dec. 11, 1973 [75] Inventor: William J.Ralston, Albuquerque, N.

Mex.

[73] Assignee: EG & G, lnc., Bedford, Mass.

[22] Filed: Dec. 13, 1972 [21] Applv No.: 314,904

[52] U.S. Cl. 340/258 C, 340/276 [51] Int. Cl. G08b 13/26 [58] Field ofSearch 340/258 C, 276

[56] References Cited UNITED STATES PATENTS 2,455,376 12/1948 Lindsay340/258 C 2,992,420 7/1961 Riker 340/258 C 3,047,849 7/1962 Hansen340/258 C 2,355,395 8/1944 Rubenstein 340/276 3,184,730 5/1965 Irish340/258 C osc PHASE REFERENCE DEMOD Primary Examiner-John W. CaldwellAssistant ExaminerGlen R. Swann, lll Att0rneyRalph L. Cadwallader et a1.

[57] ABSTRACT Each of a pair of detector wires is strung along a portionof a boundary to be protected so that both wires are exposed to similargeneral environmental conditions. The wires are interconnected inrespective resonant circuits which are similarly driven at a commonfrequency so that each resonant circuit provides an ac signal having aphase and amplitude which are variable as a function of the capacitanceof the respectivedetector wire. By synchronously demodulating thedifference between the two signals, a difference amplitude signal isobtained which is relatively sensitive to differential changes in thecapacitances. By employing the rate of change of this demodulatingdifference signal as an indication of intrusion, a system is obtainedwhich is relatively insensitive to gradual and environmental changes.

10 Claims, 5 Drawing Figures l NEGREF. OUTPUT QMENIEBBEC 1 1 ma SHEET 1BF 2 OSC POS. REF.

LATCH OUTPUT I NEGREF.

PMENIEBnEcH am I SHEET 2 CF 2 OSC 25 PHASE REFERENCE DEMOD To FILTER 33CAPACITIVE INTRUSION DETECTION SYSTEM WITH BALANCED RESONANT CIRCUITSBACKGROUND OF THE INVENTION This invention relates to intrusiondetection systems and more particularly to such systems employingcapacitive sensing elements.

While capacitive intrusion detector systems have been proposedheretofore, such systems have typically not been suitable for useout-of-doors since changing environmental conditions typically renderedit too difficult to distinguish those signals caused by an intruder fromthose occurring due to natural causes and from inherent drifting ofcomponent values in the system.

Among the several objects of the present invention may be noted theprovision of an intrusion detection system which is highly sensitive tointrusions into the protected area; the provision of such a system whichis relatively insensitive to overall changes in environmentalconditions; the provision of such a system which is relativelyinsensitive to drifting or gradual changes in local conditions; theprovision of such a system which is not subject to interference; theprovision of such a system which is highly reliable; and the provisionof such a system which is of relatively simple and inexpensiveconstruction. Other objects and features will be in part apparent and inpart pointed out hereinafter.

SUMMARY OF THE INVENTION Briefly, the intrusion detection apparatus ofthe present invention employs a pair of capacitive detector elementswhich are exposed to the environment in which intrusion is to bedetected, the capacitance with respect to ground of each element beingvariable as a function of changes in its immediate environmental area. Arespective inductance is interconnected with each detector element andforms therewith a resonant circuit, the two circuits being resonant atabout the same frequency. Means are provided for applying a preselecteddrive to each of the resonant circuits at a frequency approximatelyequal to the common half power point thereby to obtain from each circuitan a.c. signal having an amplitude and phase which are variable as afunction of the respective detector element capacitance. These a.c.signals are applied to a differential amplifier which is operative toreject common mode components of those signals and thereby obtain ana.c. difference signal. The a.c. difference signal is then synchronouslydemodulated under control of the original driving signal to obtain aphase-sensitive amplitude signal. An indication of intrusion detectionis then generated when the rate of change of the amplitude sampleexceeds a preselected level.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram ofintrusion detection apparatus in accordance with the present invention;

FIG. 2 is an illustration of the preferred method of disposing thecapacitive detector elements employed in the apparatus of FIG. 1; 7

FIG. 3 is a drawing representing the phase and amplitude response of aresonant circuit employed in the apparatus of FIG. 1;

FIG. 4 is a phase diagram useful in illustrating the mode of operationof the apparatus of FIG. 1; and

FIG. 5 is a schematic diagram illustration of an alternate embodiment ofthe invention.

Corresponding reference characters indicate corre- DESCRIPTION THEPREFERRED EMBODIMENT As indicated previously, the intrusion detectionsystem of the present invention is of the capacitive detection type,i.e., a system which detects intrusion by means of the change incapacitance induced in a detector element by the presence of theintruding object. For purposes described in greater detail hereinafter,the apparatus of the present invention employs a pair of such detectorelements, which are preferably approximately matched in electricalcharacteristics. In the preferred physical arrangement illustrated inFIG. 2, the detector elements comprise a pair of wires 11 and 13, eachof which is supported along a respective portion of a boundary withrespect to which intrusion is to be detected. In this case, the boundaryis a metallic fence, e.g. a chain link fence 15 which serves as areference ground for the system. As willbe understood by those skilledin the art, each wire will exhibit a capacitance with respect to groundand these capacitance values will be variable as a function of changesin the immediate environment adjacent each wire. In. the particularembodiment described with reference to FIG. 1, the wires 11 and 13 maybe of appreciable length, i.e., up to 200 feet each, based upon a heightabove ground of about 2 feet. The capacitance of each wire will thus beabout 440 picofarad. The detection circuitry associated with the wires11 and 13 is preferably mounted closely adjacent the wires themselves soas to minimize stray capacitances, e.g. in a weather-proof box asindicated at 16 in FIG. 2. In order to provide protection for evenlarger perimeters, multiple units may be employed,

strung end to end.In addition to acting as a ground, the

fence 15 also effectively shields the detector wires from changingconditions outside the fence which might otherwise effect the wirescapacitances.

With reference to FIG. 1, the capacitances with respect to ground of thewires 11 and .13 are indicated at C1 and C2. Interconnected with each ofthe capacitances C1 and C2 is an inductance, L1 and L2 respectively.These may be in the order of one millihenry. One end of each inductor isconnected directly to the respective detector wire while the other endof the inductor is connected to ground through a respective electrolyticcapacitor C3 or C4. With regard to the a.c. frequencies at which thedetector elements are operated, e.g. about 200 kilocycles, theelectrolytic capacitors C3 and C4 are essentially short circuits. Thus,the inductors L1 and L2 are effectively in parallel with thecapacitances C1 and C2 and form parallel resonant circuits therewith,the capacitors C3 and C4 serving only to provide d.c. isolation. Theresonant circuits are designated generally by the reference characters21 and 23.

Each of the resonant circuits can be driven by means of a signalobtained from an a.c. signal source such as stable oscillator 25. Thesignal from oscillator 25 is applied to each resonant circuit through arespective decoupling resistor R1 or R2. One of the resistors (R2) ispreferably adjustable, as indicated in the drawing, so as to permitinitial balancing of the system as described hereinafter. In addition toeffectively isolating the two resonant circuits so that they can operateindependently, the values of the resistors R1 and R2 also tend todetermine the Q of the resonant circuits. When driven in this manner,each of the resonant circuits 21 and 23 operates to provide, at thecommon junction of the resistor, inductor, and capacitor, an ac signalhaving a phase and amplitude which is variable as a function of thevalue of the respective sensing capacitance C1 or C2.

The ac signals obtained from the resonant circuits 21 and 23 are appliedto the inverting and noninverting inputs respectively of a differentialamplifier 29. As is understood by those skilled in the art, theoperation of the amplifier 29 is to reject common mode components of thesignals provided thereto and to generate an output signal which is anamplified function of the instantaneous difference between the two inputsignals supplied thereto. This a.c. difference signal is synchronouslydemodulated by a synchronous detector or demodulator 31 which isoperated under the control of the signal source oscillator 25, theoutput signal from the oscillator being applied to the detector as aphase reference signal. This synchronous demodulation yields a signalwhich is sensitive to both amplitude and phase differences between theac. signals obtained from the two resonant circuits. The demodulatedsignal is referred to herein as the difference amplitude signal. Ingeneral, the demodulator 31 is preferably of the type providing anoutput signal V K V V cos 6 Where:

Vo represents the DC output from the detector.

V V are the reference and input signals, respectively, in voltspeak-to-peak.

K is a gain constant associated with the particular detector.

6 is the phase angle between the reference and input signals in radians.

Such devices are commonly used also as phase detectors when the twoinput signals are approximately in phase-quadrature. However, asexplained hereinafter, the operation of the apparatus of the presentinvention is based upon the assumption that the input and referencesignals will be substantially phase colinear, i.e., at either 0 or l80relationship.

The difference amplitude signal is applied to a highpass, active filter33. Filter 33 employs an amplifier 35 provided with an input networkcomprising a capacitor C and a resistor R5 connected in series and afeedback network comprising a capacitor C6 and a parallel resistor R6.As will again be understood by those skilled in the art, this activefilter operates to provide a signal which is responsive to therate-of-change of the difference amplitude signal. A pair of comparatorcircuits 41 and 42 sense whether the rate-of-change signal passesoutside of a range of values between a preselected positive referencevoltage, indicated at 43, and a negative reference voltage, indicated at45. If the rate-of-change signal passes outside of the range establishedby these reference voltages, the output of the tripped comparator,operating through an AND gate 47, operates a latch circuit 49. The latchcircuit 49 provides an output signal indicating the detection of anintrusion, which signal persists until the latch circuit is reset, eventhough the rate-of-change signal returns to a value within the deadbandrange.

In accordance with one aspect of the present invention, the frequency ofthe signal provided by the oscillator 25 is selected in relation to thecharacteristics of the resonant circuits 2] and 23 so that thosecircuits are operated substantially at one of their half-pow er points,i.e., a point such that the ac. signals obtained from the resonantcircuits are phase-displaced approximately 45 with respect to thedriving signal provided by the oscillator. In FIG. 3, the amplitude andphase responses of one of the resonant circuits are represented at A andB respectively, each being plotted at a function of frequency F. Theupper and lower half-power points are indicated at D and C respectively,while the resonance frequency is indicated at R. In FIG. 4, the vectordiagram for operation around the lower half-power point is illustrated.The vector VR represents the reference signal from the source 25 whilethe vector V1 represents the signal obtained from the resonant circuit21. If the resonant circuit 23 were exactly matched with the resonantcircuit 21, subtracting its vector (V2) from the vector V1 would producean algebraic sum coincidence with the origin and thus the amplitude ofthe difference signal would be equal to zero. If, however, thecapacitance C2 is slightly increased with respect to C1, i.e., by thepresence of an intruder, the increase in capacitance will cause both anincrease in amplitude and a decrease in phase of the correspondingvector. This is indicated at V2 in FIG. 3. The difference vector VD willthus be essentially colinear with the ref erence vector VR, a conditionwhich produces the greatest sensitivity in the synchronous detector 31.

While the vector diagram of FIG. 4 facilitates visualization of theoperation of the system, it can be shown mathematically that peaks insensitivity occur at both half power points, with the lower frequencyhalf power point being slightly more sensitive than the higher frequencyhalf power point.

Summarizing, it can be seen that the system is highly sensitive todifferential changes in the capacitances exhibited by the two detectorwires 11 and 13. Thus, if an intruder should climb over the fence 15,his body capacitance would differentially disturb the balance of thesystem with a rate-of-change sufficient to exceed the acceptable limitsestablished by the filter 33 and comparators 41 and 42. Accordingly, thelatch circuit 49 would be triggered giving an intrusion indication oralarm. On the other hand, general changes in environmental conditions,such as precipitation, tend to affect both detector wires similarly sothat no alarm is given, even though the operating points of the resonantcircuits 21 and 23 may shift slightly with respect to the nominalhalf-power point. Since the sensitivity is at a maximum around thispoint the system sensitivity is not significantly affected. Further,even if general conditions tend to somewhat affect the two detectorsdifferentially, such changes will usually occur relatively gradually andthus the rate-of-change signal generated thereby will not typicallyexceed the deadband range established by the comparators and, again, nofalse indication of intrusion will be given.

In one embodiment of the system of FIG. 1, the nominal resonancefrequency of the resonant circuits 21 and 23 was about 1.52 X 10 radiansper second and the system was operated at the lower half-power point,about 1.36 X 10 radians per second. While the ac sensing voltage appliedto the connector wires was less than three volts a.c. and thus presentedno shock hazard, the

system was operative to sense an average size man at a distance of abouteight feet from the detector wire, without being prone to give falsealarms.

In the aternative embodiment illustrated in FIG. 5, the sensingcapacitances Cl and C2 are interconnected in respective resonantcircuits 61 and 62 and are driven by the oscillator 25 throughrespective isolating transistors Q1 and Q2. Although the respectiveinductances L1 and L2 are connected between the transistor collectorsand a positive voltage source, while the capacitances Cl and C2 areconnected between the respective collectors and ground, it will beunderstood that these circuit elements are effectively in parallel fora.c. signals at the frequency of the oscillator 25 and thus parallelresonance circuits are formed. Each inductance is shunted by arespective trimming capacitor C8 and C9 which allows the resonancecircuit to be trimmed so that it operates at a half-power point whendriven by the oscillator. The emitter circuit of each transistorincludes a gain controlling resistor, R8 and R9 respectively, theresistor R9 being adjustable to permit the levels of the a.c. signalsprovided by the resonant circuits 61 and 62 to be brought into initialbalance.

As in the previous embodiment, these signals are applied to differentialamplifier 29 to obtain an a.c. difference signal which is synchronouslydemodulated under the control of a phase reference signal obtaineddirectly from the oscillator 25. In this embodiment, the collectorcircuits driving the resonant circuits present a relatively highimpedance so that a higher Q can be obtained than with the resistivelydriven and isolated circuits of the embodiment of FIG. 1. As a result ofthe high Q thereby obtained, the system has a narrower responsebandwidth and is less sensitive to interfering signals in the generalfrequency range of the oscillator 25. Again, however, it is preferablethat the resonant frequency of the resonant circuits be selected inrelation to their Q and the operating frequency of the oscillator 25 sothat the resonant circuits operate nominally around a half-power point.

In view of the foregoing, it may be seen that several objects of thepresent invention are achieved and other advantageous results have beenattained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it should be understood thatall matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:

1. Intrusion detection apparatus comprising:

a pair of capacitive detector elements exposed to the environment inwhich intrusion is to be detected, the capacitance with respect toground of each element being variable as a function of changes in therespective environmental area;

interconnected with each detector element, a respective inductanceforming therewith a resonant circuit;

means for applying a preselected a.c. drive to each of said resonantcircuits thereby to obtain from each circuit an a.c. signal having anamplitude and phase which are variable as a function of the respectivedetector element capacitance, both of said resonant circuits beingdriven at the same frequency;

a differential amplifier operative to reject common mode components ofsignals applied thereto;

means for applying said a.c. signals to said amplifier thereby to obtainan a.c. difference signal;

detector means operating synchronously with said drive means fordemodulating said a.c. difference signal to obtain a phase-sensitiveamplitude signal, said amplitude signal being sensitive to intrusionsinto the environment of said detector elements.

2. Apparatus as set forth in claim 1 in which said detector elements arewires of substantially equal length supported adjacent respectiveportions of a boundary relative to which intrusion is to be detected.

3. Apparatus as set forth in claim 2 in which said wires are strung inspaced relation to a grounded metallic fence.

4. Apparatus as set forth in claim 3 in which said inductances saiddifferential amplifier and said detector means are mounted in closeproximity to said wires thereby to minimize stray capacitances.

5. Apparatus as set forth in claim 1 in which said drive means providesa phase reference signal to said detector means and in which saiddetector means is of the type providing an output signal Vo K V V cos 8where:

Vo the do. output of the detector;

K a gain constant associated with the said detector;

V the amplitude of said phase reference signal;

V the amplitude of said a.c. difference signal; and

0 the phase angle between the phase reference signal and the a.c.difference signal.

6. Intrusion detection apparatus comprising:

a pair of capacitive detector elements exposed to the environment inwhich intrusion is to be detected, the capacitance with respect toground of each element being variable as a function of changes in therespective environmental area;

interconnected with each detector element, a respective inductanceforming therewith a parallel resonant circuit resonant essentially at apreselected frequency;

means for driving each of said resonant circuits at a frequencyapproximately equal to a half-power point of said circuits, thereby toobtain from each circuit an a.c. signal having an amplitude and phasewhich are variable as a function of the respective detector elementcapacitance;

a differential amplifier operative to reject common mode components ofsignals applied thereto;

means for applying said a.c. signals to said amplifier thereby to obtainan a.c. difference signal;

detector means interconnected with said drive means and operatedsynchronously therewith for synchronously demodulating said a.c.difference signal to obtain a phase-sensitive amplitude signal; and

means providing an indication of intrusion detection when therate-of-change of said difference amplitude signal exceeds a preselectedlevel.

7. Apparatus as set forth in claim 6 in which said drive means providesa phase reference signal to said detector means and in which saiddetector means is of the type providing an output signal V0 KDVRVD C05 0where:

Vo the d.c. output of the detector;

K,, a gain constant associated with the said detector;

V the amplitude of said phase reference signal;

V the amplitude of said a.c. difference signal; and

6 the phase angle between the phase reference signal and the acdifference signal.

8. Intrusion detection apparatus comprising:

first and second detector wires, each supported along a respectiveportion of a boundary relative to which intrusion is to be detected, thecapacitance with respect to ground of each wire being variable as afunction of changes in the respective environmental area;

a respective inductance connected in parallel with the capacitanceexhibited by each wire thereby to form a circuit resonant at apreselected frequency which is common to both circuits;

resistance means for feeding each parallel circuit,

each resultant circuit having a Q which is substantially equal to apreselected value;

a constant-amplitude signal source driving both of said resonantcircuits through the respective resistance means at a frequencyapproximately equal to the lower half-power points of said resonantcircuits to obtain from each an ac signal having an amplitude and phasewhich are variable as a function of the respective detector wirecapacitances;

a differential amplifier operative to reject common mode components ofsignals applied thereto;

means for applying said a.c. signals to said amplifier thereby to obtainan ac difference signal;

detector means controlled by said signal source means for synchronouslydemodulating said a.c. difference signal to obtain a phase-sensitiveamplitude signal;

means for obtaining from said amplitude signai a rate-of-change signal;and

means providing an indication of intrusion detection when therate-of-change signal passes out of a preselected range of values.

9. Intrusion detection apparatus comprising:

first and second detector wires, each supported along a respectiveportion of a boundary relative to which intrusion is to be detected, thecapacitance with respect to ground of each wire being variable as afunction of changes in the respective environmental area;

a respective inductance connected in parallel with the capacitanceexhibited by each wire thereby to form a circuit resonant at apreselected frequency which is common to both circuits;

a transistor for feeding each parallel circuit, the parallel resonantcircuit being connected to the collector of the respective transistor;

a constant-amplitude signal source driving both of said resonantcircuits through the respective transistors at a frequency approximatelyequal to the lower half-power points of said resonant circuits to obtainfrom each an ac. signal having an amplitude and phase which are variableas a function of the respective detector wire capacitances;

a differential amplifier operative to reject common mode components ofsignals applied thereto;

means for applying said a.c. signals to said amplifier thereby to obtainan ac difference signal;

detector means controlled by said signal source means for synchronouslydemodulating said a.c. difference signal to obtain a phase-sensitiveamplitude signal;

means for obtaining from said amplitude signal a rate-of-change signal;and

means providing an indication of intrusion detection when therate-of-change signal passes out of a preselected range of values.

10. Intrusion detection apparatus comprising:

a metallic fence;

first and second detector wires, each supported adjacent a respectiveportion of said fence, the capacitance with respect to said fence ofeach wire being variable as a function of changes in the environmentadjacent that wire;

a respective inductance connected in parallel with the capacitanceexhibited by each wire thereby to form therewith a circuit which isresonant essentially at a preselected frequency, said preselectedfrequency being common to both circuits;

a constant-amplitude signal source driving both of said. resonantcircuits, through respective isolating means, at a frequencyapproximately equal to the lower half-power points of said resonantcircuits thereby to obtain from each resonant circuit an a.c. signalhaving an amplitude and phase which are variable as a function of therespective detector wire capacitances;

a differential amplifier operative to reject common mode components ofsignals applied thereto;

means for applying said a.c. signals to said amplifier thereby to obtainan ac. difference signal;

detector means controlled by said signal source means for synchronouslydemodulating said are difference signal to obtain a phase-sensitiveamplitude signal;

filter means for obtaining from said amplitude signal a rate-of-changesignal; and

means providing an indication of intrusion detection when therate-of-change signal passes out of a preselected range of values.

1. Intrusion detection apparatus comprising: a pair of capacitivedetector elements exposed to the environment in which intrusion is to bedetected, the capacitance with respect to ground of each element beingvariable as a function of changes in the respective environmental area;interconnected with each detector element, a respective inductanceforming therewith a resonant circuit; means for applying a preselecteda.c. drive to each of said resonant circuits thereby to obtain from eachcircuit an a.c. signal having an amplitude and phase which are variableas a function of the respective detector element capacitance, both ofsaid resonant circuits being driven at the same frequency; adifferential amplifier operative to reject common mode components ofsignals applied thereto; means for applying said a.c. signals to saidamplifier thereby to obtain an a.c. difference signal; detector meansoperating synchronously with said drive means for demodulating said a.c.difference signal to obtain a phasesensitive amplitude signal, saidamplitude signal being sensitive to intrusions into the environment ofsaid detector elements.
 2. Apparatus as set forth in claim 1 in whichsaid detector elements are wires of substantially equal length supportedadjacent respective portions of a boundary relative to which intrusionis to be detected.
 3. Apparatus as set forth in claim 2 in which saidwires are strung in spaced relation to a grounded metallic fence. 4.Apparatus as set forth in claim 3 in which said inductances saiddifferential amplifier and said detector means are mounted in closeproximity to said wires thereby to minimize stray capacitances. 5.Apparatus as set forth in claim 1 in which said drive means provides aphase reference signal to said detector means and in which said detectormeans is of the type providing an output signal Vo KDVRVD cos thetawhere: Vo the d.c. output of the detector; KD a gain constant associatedwith the said detector; VR the amplitude of said phase reference signal;VD the amplitude of said a.c. difference signal; and theta thE phaseangle between the phase reference signal and the a.c. difference signal.6. Intrusion detection apparatus comprising: a pair of capacitivedetector elements exposed to the environment in which intrusion is to bedetected, the capacitance with respect to ground of each element beingvariable as a function of changes in the respective environmental area;interconnected with each detector element, a respective inductanceforming therewith a parallel resonant circuit resonant essentially at apreselected frequency; means for driving each of said resonant circuitsat a frequency approximately equal to a half-power point of saidcircuits, thereby to obtain from each circuit an a.c. signal having anamplitude and phase which are variable as a function of the respectivedetector element capacitance; a differential amplifier operative toreject common mode components of signals applied thereto; means forapplying said a.c. signals to said amplifier thereby to obtain an a.c.difference signal; detector means interconnected with said drive meansand operated synchronously therewith for synchronously demodulating saida.c. difference signal to obtain a phase-sensitive amplitude signal; andmeans providing an indication of intrusion detection when therate-of-change of said difference amplitude signal exceeds a preselectedlevel.
 7. Apparatus as set forth in claim 6 in which said drive meansprovides a phase reference signal to said detector means and in whichsaid detector means is of the type providing an output signal Vo KDVRVDcos theta where: Vo the d.c. output of the detector; KD a gain constantassociated with the said detector; VR the amplitude of said phasereference signal; VD the amplitude of said a.c. difference signal; andtheta the phase angle between the phase reference signal and the a.c.difference signal.
 8. Intrusion detection apparatus comprising: firstand second detector wires, each supported along a respective portion ofa boundary relative to which intrusion is to be detected, thecapacitance with respect to ground of each wire being variable as afunction of changes in the respective environmental area; a respectiveinductance connected in parallel with the capacitance exhibited by eachwire thereby to form a circuit resonant at a preselected frequency whichis common to both circuits; resistance means for feeding each parallelcircuit, each resultant circuit having a Q which is substantially equalto a preselected value; a constant-amplitude signal source driving bothof said resonant circuits through the respective resistance means at afrequency approximately equal to the lower half-power points of saidresonant circuits to obtain from each an a.c. signal having an amplitudeand phase which are variable as a function of the respective detectorwire capacitances; a differential amplifier operative to reject commonmode components of signals applied thereto; means for applying said a.c.signals to said amplifier thereby to obtain an a.c. difference signal;detector means controlled by said signal source means for synchronouslydemodulating said a.c. difference signal to obtain a phase-sensitiveamplitude signal; means for obtaining from said amplitude signal arate-of-change signal; and means providing an indication of intrusiondetection when the rate-of-change signal passes out of a preselectedrange of values.
 9. Intrusion detection apparatus comprising: first andsecond detector wires, each supported along a respective portion of aboundary relative to which intrusion is to be detected, the capacitancewith respect to ground of each wire being variable as a function ofchanges in the respective environmental area; a respective inductanceconnected in parallel with the capacitance exhibited by each wiretherEby to form a circuit resonant at a preselected frequency which iscommon to both circuits; a transistor for feeding each parallel circuit,the parallel resonant circuit being connected to the collector of therespective transistor; a constant-amplitude signal source driving bothof said resonant circuits through the respective transistors at afrequency approximately equal to the lower half-power points of saidresonant circuits to obtain from each an a.c. signal having an amplitudeand phase which are variable as a function of the respective detectorwire capacitances; a differential amplifier operative to reject commonmode components of signals applied thereto; means for applying said a.c.signals to said amplifier thereby to obtain an a.c. difference signal;detector means controlled by said signal source means for synchronouslydemodulating said a.c. difference signal to obtain a phase-sensitiveamplitude signal; means for obtaining from said amplitude signal arate-of-change signal; and means providing an indication of intrusiondetection when the rate-of-change signal passes out of a preselectedrange of values.
 10. Intrusion detection apparatus comprising: ametallic fence; first and second detector wires, each supported adjacenta respective portion of said fence, the capacitance with respect to saidfence of each wire being variable as a function of changes in theenvironment adjacent that wire; a respective inductance connected inparallel with the capacitance exhibited by each wire thereby to formtherewith a circuit which is resonant essentially at a preselectedfrequency, said preselected frequency being common to both circuits; aconstant-amplitude signal source driving both of said resonant circuits,through respective isolating means, at a frequency approximately equalto the lower half-power points of said resonant circuits thereby toobtain from each resonant circuit an a.c. signal having an amplitude andphase which are variable as a function of the respective detector wirecapacitances; a differential amplifier operative to reject common modecomponents of signals applied thereto; means for applying said a.c.signals to said amplifier thereby to obtain an a.c. difference signal;detector means controlled by said signal source means for synchronouslydemodulating said a.c. difference signal to obtain a phase-sensitiveamplitude signal; filter means for obtaining from said amplitude signala rate-of-change signal; and means providing an indication of intrusiondetection when the rate-of-change signal passes out of a preselectedrange of values.